Support depth only pass, more thorough attachment merge checking.

[DGriffin91]

Depth only passes are useful for depth prepasses and non-vsm shadow maps.

This PR addresses two issues that were blocking using depth only passes.

• Resolver render_area() wasn't taking into account the depth attachment
• When merging passes, if the lhs candidate had less or no color attachments, it would not be evaluated against rhs. Also it would not check if the other attachment was none and Attachment::are_compatible allows one or both of the attachments to be none. I tried addressing this in Attachment::are_compatible but it seems the usage elsewhere needs to be able to allow one of the attachments to be none.

I updated the multipass example with a depth only prepass for testing. This doesn't need to be included in the PR, or could be made optional. Since this example uses the stencil buffer for layering rather than the depth buffer, further adjustments would need to be made for an appropriate example.

[DGriffin91]

Thinking about this more, maybe it would be better to just first check if they have the same total attachment quantity? Assuming that's a requirement anyway.

[DGriffin91]

Was just realizing that some of the checks at the end don't take into account that there may only be color attachments or only be depth attachments. Needs more work.